[Qemu-devel] [RFC PATCH v4 10/14] trace: Provide a per-event status define for conditional compilation

[Harsh Prateek Bora <harsh@linux.vnet.ibm.com>]

From: Lluís Vilanova <vilanova@ac.upc.edu>

NOTE: This is a port of a patch in Stefanha's tracing tree to the new
pythonic tracetool version.

Adds a 'TRACE_${NAME}_ENABLED' preprocessor define for each tracing event
in "trace.h".

This lets the user conditionally compile code with a relatively high
execution cost that is only necessary when producing the tracing
information for an event that is enabled.

Note that events using this define will probably have the "disable"
property by default, in order to avoid such costs on regular builds.

Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
Signed-off-by: Harsh Prateek Bora <harsh@linux.vnet.ibm.com>
---
 docs/tracing.txt     |   50 ++++++++++++++++++++++++++++++++++++++++++--------
 scripts/tracetool.py |    7 +++++++
 2 files changed, 49 insertions(+), 8 deletions(-)

diff --git a/docs/tracing.txt b/docs/tracing.txt
index 95ca16c..ab9048c 100644
--- a/docs/tracing.txt
+++ b/docs/tracing.txt
@@ -39,12 +39,12 @@ generate code for the trace events.  Trace events are invoked directly from
 source code like this:
 
     #include "trace.h"  /* needed for trace event prototype */
-    
+
     void *qemu_vmalloc(size_t size)
     {
         void *ptr;
         size_t align = QEMU_VMALLOC_ALIGN;
-     
+
         if (size < align) {
             align = getpagesize();
         }
@@ -98,12 +98,6 @@ respectively.  This ensures portability between 32- and 64-bit platforms.
 4. Name trace events after their function.  If there are multiple trace events
    in one function, append a unique distinguisher at the end of the name.
 
-5. If specific trace events are going to be called a huge number of times, this
-   might have a noticeable performance impact even when the trace events are
-   programmatically disabled. In this case you should declare the trace event
-   with the "disable" property, which will effectively disable it at compile
-   time (using the "nop" backend).
-
 == Generic interface and monitor commands ==
 
 You can programmatically query and control the dynamic state of trace events
@@ -227,3 +221,43 @@ probes:
                       --target-type system \
                       --target-arch x86_64 \
                       <trace-events >qemu.stp
+
+== Trace event properties ==
+
+Each event in the "trace-events" file can be prefixed with a space-separated
+list of zero or more of the following event properties.
+
+=== "disable" ===
+
+If a specific trace event is going to be invoked a huge number of times, this
+might have a noticeable performance impact even when the event is
+programmatically disabled.
+
+In this case you should declare such event with the "disable" property. This
+will effectively disable the event at compile time (by using the "nop" backend),
+thus having no performance impact at all on regular builds (i.e., unless you
+edit the "trace-events" file).
+
+In addition, there might be cases where relatively complex computations must be
+performed to generate values that are only used as arguments for a trace
+function. In these cases you can use the macro 'TRACE_${EVENT_NAME}_ENABLED' to
+guard such computations and avoid its compilation when the event is disabled:
+
+    #include "trace.h"  /* needed for trace event prototype */
+
+    void *qemu_vmalloc(size_t size)
+    {
+        void *ptr;
+        size_t align = QEMU_VMALLOC_ALIGN;
+
+        if (size < align) {
+            align = getpagesize();
+        }
+        ptr = qemu_memalign(align, size);
+        if (TRACE_QEMU_VMALLOC_ENABLED) { /* preprocessor macro */
+            void *complex;
+            /* some complex computations to produce the 'complex' value */
+            trace_qemu_vmalloc(size, ptr, complex);
+        }
+        return ptr;
+    }
diff --git a/scripts/tracetool.py b/scripts/tracetool.py
index 947b808..dc93a1a 100755
--- a/scripts/tracetool.py
+++ b/scripts/tracetool.py
@@ -123,6 +123,13 @@ def trace_h_begin(events):
 
 @for_format("h", END)
 def trace_h_end(events):
+    for e in events:
+        if 'disable' in e.properties:
+            enabled = 0
+        else:
+            enabled = 1
+        print "#define TRACE_%s_ENABLED %d" % (e.name.upper(), enabled)
+    print
     print '#endif /* TRACE_H */'
 
 
-- 
1.7.1.1